What To Know About New BA.2 Variant


What we know about BA.2 -- now the dominant cause of Covid-19 in the US


The very contagious Omicron subvariant BA.2 is now the dominant coronavirus strain in the United States, causing more than half of all Covid-19 infections last week.

The new numbers come from the CDC's genomic surveillance. The agency says that BA.2 caused between 51% and 59% of all new Covid-19 infections in the US the week ending March 26, up from an estimated 39% of all new infections the week before.

Though BA.2 is still just taking the stage in the US, it has had prominent runs in many other parts of the world, including Southeast Asia and the Western Pacific, and is winding down its European tour.

According to the World Health Organization, BA.2 is also the main cause of Covid-19 globally, out-muscling two other Omicron lineages, BA.1 and BA 1.1, to become the dominant strain. Since its takeover, international case counts have been rising again.

BA.2 infections have not reached the peaks seen with BA.1, however. Case counts appear to be leveling off in the UK, though hospitalizations and deaths are still rising.

Throughout the pandemic, the US has followed the UK by about three weeks, so when cases began rising there, health officials here took notice.

In America, BA.2 has been gaining steam since the end of January, and case numbers have plateaued. That flattening conceals regional differences. In 13 states, weekly average numbers of new cases are rising, and they have stopped falling in 14 others, according to data collected by Johns Hopkins University.

It's still not clear what this subvariant will do in the US. Even experts don't exactly know what to expect.

In Europe, you see BA.2 becoming predominant and driving a resurgence, and the likelihood that will not happen in the US is pretty low, really. Part of the reason that we're plateauing is that we're about to start going up again.

Frieden doesn't think it's a coincidence that the US Food and Drug Administration authorized additional booster shots for Americans who are 50 and older on Tuesday, the same day the CDC estimated that BA.2 was dominant.

There are a host of reasons why it's hard to know what BA.2 may do. The CDC estimates that 37 million Americans got Covid-19 over the winter, during the first Omicron wave. Many more have immunity from vaccination and boosters. Based on random blood samples, the CDC says that 95% of Americans may now have some degree of immunity from Covid-19.

Studies have determined that BA.2 evades our vaccinations about as well as the original Omicron did, so boosters are needed to restore protections against these variants. Less than half the US population 12 and older has had a recommended third dose.

Omicron threw our immune defenses for a loop. It was so different from the coronavirus strains that came before that many people who'd gotten sick with Delta or other early strains found themselves infected again.

BA.2 has about 40 amino acid changes from Omicron's BA.1, making it about as different from its cousin as Alpha, Beta and Delta were from each other. Some have wondered whether BA.2 could reinfect people who'd had BA.1.

A large study from Denmark suggests that these kinds of reinfections are possible but rare.

The research on more than 1.8 million infections found only 1,739 cases in which people tested positive for Covid-19 twice within a two-month window. Of those, 47 were BA.1 infections that were followed by BA.2.

When researchers looked more closely, they found that these types of reinfections tended to happen to young and unvaccinated people, mostly children. And their symptoms tended to be mild.

The study was posted as a preprint, meaning that it has not yet been scrutinized by outside experts and published in a medical journal.

BA.2 is exceedingly contagious. Some epidemiologists have said its basic reproduction number may be as high as 12, meaning each sick person infects an average of 12 others. That would put it on par with measles, which also spreads through the air. The basic reproduction number for BA.1 is estimated to be about 8.

Instead of going deeply into the lungs, the way Delta did, the Omicron strains seem much more focused on the upper respiratory tract, where the nose meets the back of the throat.

He thinks that because the infection concentrates there, that also helps it spread efficiently when people talk, cough or sneeze.

Perhaps one bright spot in the BA.2 picture may be severity.

Although studies in animals have suggested that BA.2 infection wasn't entirely mild, data on human infections from the UK, Denmark and South Africa shows that BA.2 isn't more likely to result in hospitalization when compared with BA.1.

This week, the UK Health Security Agency updated its data on vaccine effectiveness against BA.2. Up to 14 weeks, boosters were still 90% effective at preventing severe disease in people over the age of 65, pointing to an important way to make sure BA.2 doesn't lay us low.


Goodman, Brenda. “What We Know about BA.2 -- Now the Dominant Cause of Covid-19 in the US.” CNN, Cable News Network, 29 Mar. 2022, https://www.cnn.com/2022/03/29/health/ba-2-dominant-us/index.html. 


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